The present invention relates to automated inspection of surfaces such as printed wiring boards and the like, and more particularly to the automated high speed inspection of surfaces using a TDI sensor as a detector.
A printed wiring board (pwb) comprises a pattern of electrical conductors (made of a material such as 1.4-mil-thich copper) residing on a non-conducting substrate (made of a material such a FR-4 epoxy-fiberglass composite). During the manufacture of pwb's, the top surface of the conductive material is often intentionally roughened, in order to promote adhesion of photoresist to the conductor. Among the methods of roughening are mechanical abrasion, chemical etching, and application of a textured surface layer by electroplating (as in so-called "double-treat copper"). Each roughening method produces its own characteristic surface texture.
It is therefore a requirement, in the design of a machine for the optical inspection of pwb's that the machine be able to cope effectively with a wide variety of surface textures. It is also desirable, in order that the machine be as flexible in application as possible, that it be able to correctly inspect pwb's in which the conductors have smooth surfaces.
The most common and straightforward way to illuminate opaque optical surfaces for inspection is to provide illumination through the same lens that will be used to view the inspected surface, and to collect with that lens the light which is reflected or scattered from the surface. This method is commonly known as brightfield vertical illumination, or simply as brightfield illumination.
FIG. 2 illustrates the problem which is inherent in using brightfield illumination to inspect pwbs. A copper conductor 8 (shown in cross-section) resides on an insulating substrate 9. The top surface of conductor 8 is shown to be rough (the characteristic dimension of the roughness is greatly exaggerated for illustrative purposes). Illumination is provided through lens 11, which is also used for viewing light reflected or scattered from the surface.
Consider now the behavior of this system in inspecting a particular point 13 on the conductor surface. Point 13 has been selected, for illustrative purposes, to be within a small area that is sloped substantially away from level. Illumination rays 1 and 2 arrive at point 13 from the extreme edges of lens 11. All other light rays striking point 13 will arrive at angles between rays 1 and 2. The inclination of the surface of point 13 is such that ray 1 is reflected into ray 3, and ray 2 is reflected into ray 4, both of these rays lying outside the aperture of lens 11. All other illumination rays will reflect somewhere between ray 3 and ray 4, which is to say that none of the illuminating light will be reflected back into lens 11. Any optical sensor that is placed above lens 11, so as to view the returning rays, will see point 13 as being black, because none of the light leaving point 13 gets through the lens.
The general point being illustrated here is that when a rough surface is viewed by brightfield vertical illumination, the steeply inclined portions of that surface will tend to appear dark, and the overall appearance of the surface will be strongly mottled.
It is necessary for the optical inspection machine to distinguish between regions of copper and regions of insulator. This is often done by taking advantage of the fact that conductive regions are more reflective than are insulating regions, at least at selected wavelengths. Electronic logic can be employed which identifies dark regions are being insulative and bright regions as being conductive. If an optical illumination system causes conductive regions to appear mottled, then some parts of the conductive regions will be falsely identified as being insulative.
A known cure for this problem is to average observed reflectance values over a relatively large region, so as to take advantage of the fact that the average reflectance, even of rough-textured copper, is often higher than the average reflectance of substrate materials. This method has the disadvantage, however, that it makes it impractical to detect actual missing-copper defects of a size smaller than the averaging region.
Defining Numerical Aperture (NA) of the illuminator in the conventional way, which is to say that NA=sin (.theta.), .theta. being the angle between a normal to the surface and the extreme illuminating ray, the NA of illumination should be at least about 0.7 NA, and preferable greater than 0.8 NA. In addition, the illumination should be of constant intensity (watts/steradian/cm.sup.2) over all angles of incidence (i.e. quasi-Lambertian).
An achievement of the present invention is that, be reducing optically the apparent mottling of rough-textured surfaces, it is made possible to avoid large-area averaging, and consequently it becomes possible to detect smaller regions of missing conductive material.
It is not new to provide focussed illumination from a large range of angles, up to a numerical aperture which may even exceed 0.9. Such illumination is achieved, for example, brightfield vertical illuminators used in high-magnification microscopes employing high-NA objectives. The best of such microscopes achieve illumination NA's on the order of 0.95. The intensity of illumination in such microscopes is not, however, independent of the angle of incidence. The falloff in transmission of strongly-curved lens elements at large angles causes the illumination provided by such objectives to be significantly weaker at angles far from the normal.